1. Field of the Invention
The present invention relates to a clear coating composition for an automobile and to a method of forming a multi-layer coating film using the same.
2. Description of the Related Art
As a binder to be used for the top coat for an automobile, a hydroxyl group-containing polymer and a melamine polymer curing agent are commonly used in combination. For example, Japanese Patent Application Laid-Open Gazette No. Hei 5-202335 (JP 5-202335 A) describes a thermosetting coating composition containing hydroxyl group-containing acrylic resin, alkyl ether melamine resin, polyfunctional block isocyanate, and a phosphite compound. However, generally speaking, a cured coating film obtained by using such a melamine polymer as the curing agent exhibits poor acid resistance and is particularly susceptible to degradation due to acid rain which has become a problem in recent years, causing a problem in terms of the external appearance. Since the poor acid resistance of a coating film obtained by using a melamine polymer as the curing agent is attributable to the triazine ring in the melamine polymer, the problem of poor acid resistance is not solved as long as the melamine polymer is used as the curing agent.
In order to overcome this problem, Japanese Patent Application Laid-Open Gazette No. Hei 2-45577 (JP 2-45577 A) and Japanese Patent Application Laid-Open Gazette No. Hei 3-287650 (JP 3-287650 A) each describe a coating composition not using a melamine polymer. Since such a coating composition uses as the cross-linking point ester linkage produced by reacting a carboxylic acid group and an epoxy group with each other, the coating composition exhibits good acid resistance while having sufficient weatherability as the top coating film for an automobile.
Further, Japanese Patent Application Laid-Open Gazette No. Hei 9-132753 (JP 9-132753 A) describes a thermosetting coating composition using both block isocyanates (A) and (B) Block isocyanate (A) contains hydroxyl group-containing resin and block isocyanate as major components, and as the block isocyanate, having isophorone diisocyanate as an isocyanate component. Block isocyanate (B) contains an adduct of hexamethylenediisocyanate or its isocyanurate compound to a lactone modified polyhydric alcohol as an isocyanate component. However, even the coating film obtained from the thermosetting coating composition described in JP 9-132753 A still proves insufficient in terms of its flaw resistance, especially its scratch resistance.
Further, Japanese Patent Application Laid-Open Gazette No. 2003-253191 (JP 2003-253191 A) describes a clear coating composition composed of: a half-ester acid group-containing acrylic copolymer; an epoxy group-containing acrylic copolymer; and at least one carboxyl group-containing polymer selected from the group consisting of a carboxyl group-containing polyester polymer and a carboxyl group-containing acryl polymer, the clear coating composition including a predetermined amount of soft segment part represented by the following formula: —(CH2)n— (wherein n represents an integer of 4 or more). Such a clear coating composition can provide a coating film having well balanced physical properties in all the aspects of scratch, acid, and solvent resistances.